East African climate variability on different time scales : the Suguta Valley in the African-Asian

153 Pages·2011·25.03 MB·English

by  Annett Junginger

Institut für Erd- und Umweltwissenschaften
Mathematisch-Naturwissenschaftliche Fakultät
Universität Potsdam
East African Climate Variability on Different Time Scales
The Suguta Valley in the African-Asian Monsoon Domain
Annett Junginger
Kumultative Dissertation zur Erlangung des akademischen Grades
„doctor rerum naturalium (Dr. rer. nat.) in der Wissenschaftsdisziplin „Geologie“
Potsdam, im November 2011
Selbständigkeitserklärung
Hiermit versichere ich, die vorliegende Arbeit selbstandig und nur unter Verwendung der angegebenen Quellen und Hilfsmittel
angefertigt zu haben.

Declaration of independent work
Hierwith I declar that I have written this paper on my own, distinguished citations, and used no other than the named sources and
aids.

Potsdam, im November 2011

Online veröffentlicht auf dem
Publikationsserver der Universität Potsdam:
URL http://opus.kobv.de/ubp/volltexte/2011/5683/
URN urn:nbn:de:kobv:517-opus-56834
http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-56834
Dissertation
East African Climate Variability on Different Time Scales
The Suguta Valley in the African-Asian Monsoon Domain
Annett Junginger
Table of Contents
Abstract …………………………………………………………………………………………………… 09
Zusammenfassung ……………………………………………………………………………………… 11
Chapter 1 - Introduction into this doctoral thesis ……………………………………………………… 15
1.1 General Introduction …………………………………………………………………………… 16
1.2 The Suguta Valley - a general overview ………………………………………………………… 21
1.2.1 Geographical setting of the Suguta Valley ………………………………………………… 21
1.2.2 Geological development of the Suguta Valley …………………………………………… 22
1.2.3 Present climate in the Suguta Valley ……………………………………………………… 23
1.2.4 Climate variability in East Africa …………………………………………………………… 25
1.2.5 The Suguta Valley and its importance for palaeo-climatic studies ……………………… 29
1.3 References ………………………………………………………………………………………… 31
Chapter 2 - Solar Variations and Holocene East African Climate ……………………………………… 39
2.1 Solar Variations and Holocene East African Climate …………………………………………… 40
Abstract …………………………………………………………………………………………… 40
2.1.1 Introduction ………………………………………………………………………………… 40
2.1.2 Setting ……………………………………………………………………………………… 40
2.1.3 Present day climate ………………………………………………………………………… 41
2.1.4 Methods …………………………………………………………………………………… 41
2.1.5 Results ……………………………………………………………………………………… 41
2.1.6 Discussion & Conclusions ………………………………………………………………… 42
2.1.6.1 Onset of the AHP …………………………………………………………………… 42
2.1.6.2 Internal variability of the AHP ……………………………………………………… 44
2.1.6.3 Termination of the AHP ……………………………………………………………… 46
2.1.7 Summary …………………………………………………………………………………… 47
2.2 Supplementary information 1 - Lacustrine sediment investigations ………………………… 48
2.2.1 Methods …………………………………………………………………………………… 48
2.2.1.1 Materials ……………………………………………………………………………… 48
2.2.1.2 Age control …………………………………………………………………………… 48
2.2.1.3 Grain-size and magnetic susceptibility ……………………………………………… 50
2.2.1.4 Geochemical analyses ……………………………………………………………… 50
2.2.1.5 Occurrence of of Melanoides Tuberculata …………………………………………… 50
2.2.2. Results ……………………………………………………………………………………… 51
2.2.2.1 Chronology …………………………………………………………………………… 51
2.2.2.2 Description of investigated lake sediment sections ………………………………… 51
2.2.3 Interpretation of lake-sediment sections ………………………………………………… 57
2.2.3.1 Water level above sediments ………………………………………………………… 57
2.2.3.2 Geochemical investigations ………………………………………………………… 57
2.2.4 Conclusion ………………………………………………………………………………… 58
2.3 Supplementary information 2 - Lake-balance modelling …………………………………… 59
2.3.1 Methods …………………………………………………………………………………… 59
2.3.1.1 Description of the model …………………………………………………………… 59
2.3.1.2 Initialization of the model …………………………………………………………… 59
2.3.2 Results ……………………………………………………………………………………… 61
2.3.2.1 Modelling precipitation for the lake highstand …………………………………… 61
2.3.2.2 Modelling abrupt and gradual changes …………………………………………… 61
2.3.3 Interpretation of the modelling results …………………………………………………… 61
2.3.4 Conclusions ………………………………………………………………………………… 62
2.4 Supplementary information 3 - Final lake-level reconstructions & interpretation ………… 64
2.4.1 Method on completing lake level reconstruction ………………………………………… 64
2.4.2 Results and interpretation ………………………………………………………………… 64
2.4.2.1 Onset of the AHP …………………………………………………………………… 64
2.4.2.2 Younger Dryas (YD) ………………………………………………………………… 64
2.4.2.3 Internal lake-level fluctuations during the AHP …………………………………… 65
2.4.2.4 The 8.2 ka event in the Suguta Valley ……………………………………………… 67
2.4.2.5 Termination of the AHP in the Suguta Valley ……………………………………… 68
2.4.3 ENSO influence in the Suguta Valley ……………………………………………………… 68
2.4.4 Conclusions ………………………………………………………………………………… 68
2.5 Acknowledgements ……………………………………………………………………………… 69
2.6 References ………………………………………………………………………………………… 70
Chapter 3 - Environmental variability in Lake Naivasha, Kenya, over the last two centuries ……… 75
3.1 Environmental variability in Lake Naivasha, Kenya, over the last two centuries …………… 76
Abstract …………………………………………………………………………………………… 76
3.1.1 Introduction ……………………………………………………………………………… 76
3.1.2 Study site …………………………………………………………………………………… 77
3.1.3 Material and Methods ……………………………………………………………………… 78
3.1.3.1 Core sampling ……………………………………………………………………… 78
3.1.3.2 Chronology (210Pb) …………………………………………………………………… 78
3.1.3.3 Sedimentology and Geochemistry ………………………………………………… 79
3.1.3.4 Diatom counts and Transfer Functions ……………………………………………… 79
3.1.4 Results ……………………………………………………………………………………… 80
3.1.4.1 Chronology (210Pb) …………………………………………………………………… 80
3.1.4.2 Lithology and Geochemistry ………………………………………………………… 81
3.1.4.3 Diatom assemblages ………………………………………………………………… 81
3.1.4.4 Statistical analysis and environmental reconstruction ……………………………… 83
3.1.5 Discussion …………………………………………………………………………………… 83
3.1.5.1 Zone I (ca. 1820 to 1896 AD) ………………………………………………………… 84
3.1.5.2 Zone II (ca. 1896 to 1938 AD) ………………………………………………………… 87
3.1.5.3 Zone III (ca. 1938 to 2007 AD) ……………………………………………………… 87
3.1.6 Summary and Conclusions ………………………………………………………………… 88
3.2 Acknowledgements ……………………………………………………………………………… 89
3.3 References ……………………………………………………………………………………… 90
Chapter 4 - Contributions to climate proxy sites in the East African Rift ……………………………… 95
4.1 Contributions to climate proxy sites in the East African Rift ………………………………… 96
Abstract …………………………………………………………………………………………… 96
4.1.1 Introduction ………………………………………………………………………………… 96
4.1.2 Setting ……………………………………………………………………………………… 98
4.1.2.1 Climatic setting of East Africa ……………………………………………………… 98
4.1.2.2 Topography …………………………………………………………………………… 100
4.1.3 Materials and Methods …………………………………………………………………… 100
4.1.4 Results and Discussion ……………………………………………………………………… 102
4.1.4.1 IOD/ENSO & the 11-year solar cycle 1996-2010 …………………………………… 102
4.1.4.2 Seasonal lake basin precipitation …………………………………………………… 102
4.1.4.2.1 West-Ethiopian Plateau – Lake Tana …………………………………………… 102
4.1.4.2.2 Central-Ethiopian Plateau – Rift basins Abaya, Awassa, Ziway ……………… 109
4.1.4.2.3 Lakes in between two plateaus - Lake Suguta & Turkana …………………… 112
4.1.4.2.4 Central East African Plateau – Baringo, Nakuru, Naivasha …………………… 114
4.1.4.2.5 Southern East African Plateau Lakes - Natron and Manyara ………………… 118
4.1.5 Preliminary summary and conclusions …………………………………………………… 119
4.2 Supplementary information ……………………………………………………………………121
4.2.1 - The coupling and dependency of IOD and ENSO – a literature overview ……………… 121
4.2.2 - Additional information about solar influences to IOD and ENSO ……………………… 121
4.3 Acknowledgements ………………………………………………………………………………122
4.4 References …………………………………………………………………………………………123
Chapter 5 - Summary, Conclusions, Outlook ……………………………………………………………127
5.1 Summary and Conclusions ………………………………………………………………………128
5.2 Outlook ……………………………………………………………………………………………130
Chapter 6 - Acknowledgements …………………………………………………………………………133
Chapter 7 - Appendix ……………………………………………………………………………………137
Appendix 01 - Climatology in The Suguta catchment area ………………………………………138
Appendix 02 - Data for BG08 ………………………………………………………………………139
Appendix 03 - Data for EL08 …………………………………………………………………………141
Appendix 04 - Data for LN08 …………………………………………………………………………142
Appendix 05 - Palaeo-lake level curve for Suguta Valley …………………………………………144
Appendix 06 - Palaeo-lake levels Africa ……………………………………………………………145
Appendix 07 - 200-year Sedimentology and diatom investigation for Lake Naivasha …………146
Appendix 08 - Instrumental lake level data main lake Naivasha (S. Higgens) ……………………147
Appendix 09 - Lake lake reconstruction of Crescent Island Crater ………………………………152

Abstract
Societal and economic needs of East Africa rely entirely on the availability of water, which is governed by the
regular onset and retreat of the rainy seasons. Fluctuations in the amounts of rainfall, as it is a permanent
feature in East Africa for at least the past 15,000 years, has tremendous impact causing widespread famine,
disease outbreaks and human migrations. Efforts towards high resolution forecasting of seasonal precipi-
tation and hydrological systems are therefore needed, which requires high frequency short to long-term
analyses of available climate data that I am going to present in this doctoral thesis.
Long-term studies | The nature and causes of intensity variations of the African and Indian summer mon-
soons during the African Humid Period (AHP, 14,800 - 5,500 years BP, BP = before present), especially their
exact influence on regional climate relative to each other, is currently intensely debated. As an example, no
consensus exists concerning the abrupt vs. gradual onset and termination of this event, as well as the char-
acter and style of the internal climate variability during the AHP. The main part of this doctoral thesis pre-
sents a well 14C-dated, reservoir-corrected, lake-level record from the remote Suguta Valley in the northern
Kenya Rift for the AHP. The reconsruction of water level changes is based on the combined analysis of dated
shoreline elevations, sediment compositions and hydrological modelling. The results uncovered that during
the AHP only 26% of additional rainfall caused the presently desiccated valley to be filled by a 300 m deep
and 2,200 km2 large palaeo-lake that was, due to its extreme catchment size of 13,000 km2 and amplifier-lake
characteristic, highly sensitive to relatively moderate climate changes. This record from the Suguta Valley,
located between the West-African and Indian Monsoon systems, explains very deep lakes in the East African
Rift during the AHP as an indirect consequence of a strengthened Indian Summer Monsoon (ISM) during
the precessional forced insolation maximum over the northern hemisphere. Besides the general enhanced
atmospheric moisture availability, a possible deepening of the Tibetan Low due to higher insolation was
responsible for a larger atmospheric pressure gradient between East Africa and India that caused the longi-
tudinal shift of the Congo Air Boundary (CAB) eastwards over the East African Plateau. During the generally
wet AHP, minor humidity changes causing abrupt lake level fluctuations of up to 100 m in less than 100
years are explained by small-scale solar irradiation changes that have caused a reversal of the process used
to explain the AHP. Instead, the termination of the AHP occurred nonlinearly. The change towards an equa-
torial insolation maximum ca. 6.5 ka ago in the course of the earth‘s precession cycle caused a prolonged
intense West-African monsoon whereas the ISM weakened and thus the pressure gradient necessary to pull
the CAB over the East African Plateau. This change has led to a 1000-year earlier termination of the East Afri-
can Plateau lakes as the result of missing CAB related rainfall, in contrast to the Ethiopian Rift lakes including
Lake Turkana and West Africa. The study also explains, that topographic barriers play an important role in
defining the response of large lakes to climate change during the AHP in the context of monsoon instabili-
ties due to solar radiation fluctuations and associated displacements of circulation systems.
200 years | The second part of the thesis is on testing these relationships between climate forcing and
lake hydrology for shorter timescales during different climatic preconditions. The results of a 200 year-old
sediment core study from Lake Naivasha in the Central Kenya Rift, one of the very few present freshwater
lakes in East Africa, revealed and confirmed, that the appliance of proxy records for palaeo-climate recon-
struction for the last 100 years within a time of increasing industrialisation and therefore human impact to
the proxy-record containing sites are broadly limited. Since the middle of the 20th century, intense anthro-
pogenic activity around Lake Naivasha has led to cultural eutrophication, which has overprinted the influ-
ence of natural climate variation to the lake usually inferred from proxy records such as diatoms, transfer-
functions, geochemical and sedimentological analysis as used in this study. The results clarify the need for
proxy records from remote unsettled areas to contribute with pristine data sets to current debates about
anthropologic induced global warming since the past 100 years.
14 years | In order to avoid human influenced data sets and validate spatial and temporal heterogeneities of
proxy-records from East Africa, the third part of the thesis therefore concentrated on the most recent past
Abstract 9
14 years (1996-2010) detecting climate variability by using remotely sensed rainfall data. The advancement
in the spatial coverage and temporal resolutions of rainfall data allow a better understanding of influencing
climate mechanisms and help to better interpret proxy-records from East Africa in order to reconstruct past
climate conditions. The study focuses on the dynamics of intraseasonal rainfall distribution within catch-
ments of eleven lake basins in the East African Rift, namely Ziway, Awassa, Abaya, Turkana, Suguta, Baringo,
Nakuru, Naivasha, Natron, Manyara and Tana often used for palaeo-climate studies. We discovered that rain-
fall in adjacent basins exhibits various amplitudes in intraseasonal variability, showing biennial to triennial
precipitation patterns and even do not necessarily correlate often showing opposite trends. The variability
among the watersheds is driven by the complex interaction of topography, in particular the shape, length
and elevation of the catchment and its relative location to the East African Rift System and predominant
influence of one of the main convergence zones ITCZ or CAB. The location and intensities of these conver-
gence zones are dependent on low developments over India, sea surface temperature variations in the
Atlantic, Pacific or Indian Ocean, Quasibiennial Oscillation phases and the 11-year solar cycle. Among all
seasons we observed that July to September is the season of highest complex rainfall variability, especially
for the East African Plateau basins most likely due to the irregular penetration and sensitivity of the CAB.
These findings confirm conclusion from the main part of this doctoral thesis that large lakes in the early to
mid-Holocene AHP in equatorial East Africa were directly related to a rather regular contribution of CAB
related rainfalls to the studied area, but due to high topography sensitive to changes in solar irradiation and
earth’s precession.
10 Abstract